If you make unlimited storage space available to users, your users will use unlimited storage space...and then come back and ask for more storage space to be made available.
- Avoid giving out unlimited disk space or mailbox space to your users, computer systems in general are a limited resource.  It's always easier to give than to take, especially when it comes to increasing or decreasing mailbox quota or file server diskspace quota.  Users and admins may not like quotas but at the very least it requires people to present a business reason for increased storage requirements.

If you don't clean out your garbage from your storage, your storage space will eventually fill up with garbage.
- Take a hint from the nearest public library which has been dealing with this problem for ages, defining and implementing a proper data pruning procedure is a must to avoid filling up the storage sheds with redundant or irrelevant garbage.  Imagine a library that would never throw away any book it receives...

Define where your critical data is and make sure it is backed up.
- Critical data can exist in a lot of different places; mailboxes, file servers, AD, etc.  Periodically assessing whether new critical points have been added is a must as computer systems are dynamic and prone to changes.  Redundant copies of backups are also good to have, it's always cheaper to have one backup too many than one too few.  Complementing different types of backups is also good to minimize the possibility of one backup application failing.

Verify your disaster recovery plans
- Backing up data is one thing, verifying it is another matter....without verification you have no guarantee that you can recover from a disaster scenario.  There's nothing worse than being lulled into a false sense of security and then discovering at 03:00 in the morning that you weren't actually backing up that critical piece of information that you now need to restore or that the format in which you were backing it up is unusable for a restore.